Microphone arrays having directional patterns can be made using two or more spaced, omnidirectional microphones. Systems using two microphones to form first order directional patterns are in widespread use in hearing aids today. The directional performance can theoretically be improved by using three or more microphones to form second order, or other higher order, directional patterns. These second and higher order directional systems, however, are made more difficult by the practical issue that the microphone sensitivities must be matched very closely to obtain the improved directional performance. Methods are needed to match the microphones sensitivity as well as is possible, and also to obtain improved directionally in the presence of the remaining sensitivity errors.
Attempts have been made to measure phase differences of microphones at frequencies just below the resonant frequency of the microphones, and to only accept a group of microphones for an array having such phase differences within a predetermined tolerance. Such attempts have been too restrictive in finding microphones which fall within this criteria, while at the same time such attempts have still not determined adequately matched microphones.
With the assumption that the microphones are not perfectly matched, there is also a need to determine in what order to place the microphones in the array for optimum directivity.
The present invention is provided to solve these and other problems.